Problem with the weight paint mode when there are a lot of geometry on the mesh
in Weight Paint mode press M or V to enable Face or Vertice selection masking (you will notice one of the button at the bottom of 3D View will get pressed). after this you can either use simplified selection tools in Weight Paint mode itself or go to Edit mode to use a full specter of selection tools there and then return back to Weight Paint mode with your selection preserved between modes.